Vehicle engine systems may include various vacuum consumption devices that are actuated using vacuum. These may include, for example, a brake booster. Vacuum used by these devices may be provided by a dedicated vacuum pump, such as an electrically-driven or engine-driven vacuum pump. While such vacuum pumps advantageously produce a pumping curve that is independent of intake manifold pressure, they do so at the expense of fuel and/or energy efficiency. As an alternative to such resource-consuming vacuum pumps, one or more aspirators may be coupled in an engine system to harness engine airflow for generation of vacuum. Aspirators (which may alternatively be referred to as ejectors, venturi pumps, jet pumps, and eductors) are passive devices which provide low-cost vacuum generation when utilized in engine systems. An amount of vacuum generated at an aspirator can be controlled by controlling the motive air flow rate through the aspirator. For example, when incorporated in an engine intake system, aspirators may generate vacuum using energy that would otherwise be lost to throttling, and the generated vacuum may be used in vacuum-powered devices such as brake boosters.
While aspirators may generate vacuum at a lower cost and with improved efficiency as compared to vacuum pumps, their use in engine intake systems has traditionally been constrained by intake manifold pressure. Whereas conventional vacuum pumps produce a pumping curve which is independent of intake manifold pressure, pumping curves for aspirators arranged in engine intake systems may be unable to consistently provide a desired performance over a range of intake manifold pressures. Some approaches for addressing this issue involve arranging a valve in series with an aspirator, or incorporating a valve into the structure of an aspirator. An opening amount of valve is then controlled to control the motive air flow rate through the aspirator, and thereby control an amount of vacuum generated at the aspirator. By controlling the opening amount of the valve, the amount of air flowing through the aspirator and the air flow rate can be varied, thereby adjusting vacuum generation as engine operating conditions such as intake manifold pressure change. However, such valves can add significant component and operating costs to engine systems. As a result, the cost of including the valve may reduce the advantages of aspirator vacuum control.
To address at least some of these issues, the inventors herein have identified a parallel, valved aspirator arrangement which, when incorporated in an engine system, may advantageously produce a pumping curve comparable to that of a conventional driven vacuum pump without the costs and efficiency losses of a conventional vacuum pump. For example, the inventors herein have recognized that the valves of multiple valved aspirators arranged in parallel and bypassing an intake throttle may be controlled based on intake manifold vacuum and/or based on desired engine airflow to minimize throttling losses while generating vacuum for use with vacuum-powered devices. Because multiple, parallel aspirators are used, each aspirator may have a relatively small flow diameter and yet the arrangement can still achieve an overall motive flow rate commensurate with that of a single larger aspirator when needed. The relatively small flow diameters of the aspirators enable the use of smaller, cheaper valves controlling their motive flow. Further, relative flow diameters of the parallel aspirators may be strategically selected such that the valves of the aspirators may be controlled based on intake manifold vacuum level and/or desired engine airflow to produce a desired pumping curve. Furthermore, because the combined motive flow rate through the aspirator arrangement is controllable via the valves, conditions where the motive flow through the aspirators may cause air flow greater than desired may be reduced. Thus, since air flow rate greater than desired can lead to extra fuel being injected, fuel economy may be improved by use of the aspirator arrangement.
In one example, a method for an engine includes increasing a combined motive flow rate through a parallel aspirator arrangement of at least two valved aspirators bypassing an intake throttle as intake manifold pressure increases. This method takes advantage of the engine's ability to handle a greater throttle bypass flow rate as intake manifold pressure increases by controlling the valves of the valved aspirators of the aspirator arrangement such that the combined motive flow rate through the aspirator arrangement increases with increasing intake manifold pressure. When the combined motive flow rate through the aspirator arrangement increases, it follows that the vacuum generated by the aspirator arrangement increases, and therefore a pumping curve which resembles a vacuum pump's pumping curve (e.g., which is independent of intake manifold) may be achieved by the aspirator arrangement. Accordingly, the technical result achieved via this example method is the generation of vacuum by a parallel valved aspirator arrangement in quantities that are substantially independent of intake manifold pressure, while continuing to supply an appropriate engine air flow rate. In embodiments where the aspirator arrangement bypasses the intake throttle, the intake throttle may be adjusted to supply a difference between a desired engine air flow rate and a maximum combined motive flow rate through the aspirator arrangement.
Further, the inventors herein have recognized that the parallel valved aspirator arrangement described herein may advantageously supply a sufficient, controllable engine air flow rate during intake throttle fault conditions. Accordingly, a cheaper intake throttle may be used instead of a more costly intake throttle with a partially open unpowered position which is often used in engine systems to allow for sustained engine operation in the case of malfunction of electronic throttle control.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.